staging: ath6kl: Cast variable to size_t to avoid compile warning

The min() macro does strict type-checking so use min_t() instead
to silence a compile warning.

Cc: Naveen Singh <naveen.singh@atheros.com>
Signed-off-by: Javier Martinez Canillas <martinez.javier@gmail.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>